Cellular systems utilize a plurality of base stations to connect user terminals, such as cellular telephones, to the cellular system. Each of the user terminals initially communicates with a base station to exchange information with the system. The user terminals often must be subsequently handed over, however, to other base stations when the user terminals physically move or the current base station experiences congestion while servicing other user terminals.
In advanced cellular systems, several aspects of the communication link between a base station and a user terminal vary with the location of the user terminal. Examples of parameters that are location-dependent include modulation level and coding rate, transmit power, and timing advance for uplink transmissions.
Normally, the parameters that are appropriate for a particular user terminal must be determined by a process that includes probing the channel conditions, reporting the results, and facilitating a signaling/negotiation of the values between the base station and the user terminal. Unfortunately this process typically uses some of the channel capacity that could otherwise be used for carrying data traffic. In addition, in a packet data system, terminals typically use the system in a bursty fashion rather than continuously. As a result, user terminals may move or go into a power-saving mode between transmissions and may need to re-establish the appropriate link parameters on a frequent basis, which further reduces the data capacity and/or increases the latency of the system.
Furthermore, the performance of handoffs of a user terminal from one base station to another is typically dependent on a so-called neighbor list of potential servicing base stations that have been measured. An inaccurate neighbor list can degrade service quality (including bit rate, latency, or voice quality) or lead to a dropped call. If the size of the neighbor list may vary, a larger neighbor list can increase the volume of measurements required in order to make a handoff, leading to an increased latency and/or decreased battery life for the user terminals. Also, a neighbor list typically offers inadequate and outdated information upon startup or upon emergence from a power savings (e.g., sleep) mode, a frequent occurrence in a packet data system.
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